Changing the state of an object is known as an event. For example, click on button, dragging mouse etc. The java.awt.event package provides many event classes and Listener interfaces for event handling.
| Event classes | Listener Interfaces |
|---|---|
| ActionEvent | ActionListener |
| MouseEvent | MouseListener and MouseMotionListener |
| MouseWheelEvent | MouseWheelListener |
| KeyEvent | KeyListener |
| ItemEvent | ItemListener |
| TextEvent | TextListener |
| AdjustmentEvent | AdjustmentListener |
| WindowEvent | WindowListener |
| ComponentEvent | ComponentListener |
| ContainerEvent | ContainerListener |
| FocusEvent | FocusListener |
Steps to perform Event Handling:
Following steps are required to perform event handling:
- Implement the Listener interface and overrides its methods
- Register the component with the Listener. For registering the component with the Listener, many classes provides the registration methods. For example:
- Button : public void addActionListener(ActionListener a){}
- MenuItem : public void addActionListener(ActionListener a){}
- TextField : public void addActionListener(ActionListener a){} public void addTextListener(TextListener a){}
- TextArea public void addItemListener(ItemListener a){}
- List : public void addActionListener(ActionListener a){}
public void addItemListener(ItemListener a){} - Event Handling Codes:
We can put the event handling code into one of the following places: - Same class
- Other class
- Anonymous class
ActionEvent
| Event | Fired by | Listener interface | Handler method |
|---|---|---|---|
| java.awt.event.ActionEvent | JButton JCheckBoxMenuItem JComboBox JFileChooser JList JRadioButtonMenuItem JTextField JToggleButton |
ActionListener | actionPerformed() |
Example of ActionEvent
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class AEvent extends JFrame implements ActionListener
{
JTextField tf;
AEvent()
{
tf = new JTextField();
tf.setBounds(60,50,170,20);
JButton b = new JButton("click me!");
b.setBounds(100,100,100,30);
b.addActionListener(this);
add(b);
add(tf);
setSize(400,300);
setTitle("ActionEvent Example");
setLayout(null);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);
}
public void actionPerformed(ActionEvent e)
{
tf.setText("Welcome..");
JButton b1 = new JButton("Don't click");
add(b1);
b1.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent ae)
{
JTextField tf1 = new JTextField();
tf1.setBounds(60,160,250,30);
tf1.setForeground(Color.red);
tf1.setFont(new Font("serief",Font.BOLD,15));
tf1.setText("Welcome to hungry4java.blogspot.com");
add(tf1);
}
});
b1.setBounds(100,200,100,30);
}
public static void main(String[] args)
{
new AEvent();
}
}
ItemEvent
| Event | Fired by | Listener interface | Handler method |
|---|---|---|---|
| java.awt.event.ItemEvent | JCheckBoxMenuItem JComboBox JRadioButtonMenuItem JToggleButton |
ItemListener | ItemStateChanged() |
Example of ItemEvent
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class ItemEventDemo
{
JFrame f;
ItemEventDemo()
{
f = new JFrame("Combo Example");
String country[] = {"yellow","cyan","blue","red"};
final JComboBox cb = new JComboBox(country);
cb.setBounds(50,50,90,20);
f.add(cb);
f.setLayout(null);
f.setSize(400,500);
f.setVisible(true);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
cb.addItemListener(new ItemListener()
{
public void itemStateChanged(ItemEvent e)
{
int index = cb.getSelectedIndex();
if(index == 0)
f.getContentPane().setBackground(Color.yellow);
if(index == 1)
f.getContentPane().setBackground(Color.cyan);
if(index == 2)
f.getContentPane().setBackground(Color.blue);
if(index == 3)
f.getContentPane().setBackground(Color.red);
}
});
}
public static void main(String[] args)
{
new ItemEventDemo();
}
}
AdjustmentEvent
| Event | Fired by | Listener interface | Handler method |
|---|---|---|---|
| java.awt.event.AdjustmentEvent | JScrollBar | AdjustmentListener | adjustmentValueChanged() |
Example of AdjustmentEvent
import java.applet.*;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class AdjustmentEventDemo extends JFrame implements AdjustmentListener
{
JScrollBar r,g,b;
JPanel p;
AdjustmentEventDemo()
{
setLayout(null);
p = new JPanel();
p.setBounds(50,50,500,60);
r = new JScrollBar(0,1,20,1,275);
g = new JScrollBar(0,1,20,1,275);
b = new JScrollBar(0,1,20,1,275);
p.setLayout(new GridLayout(3,1));
p.add(r);
p.add(g);
p.add(b);
add(p,"North");
r.addAdjustmentListener(this);
g.addAdjustmentListener(this);
b.addAdjustmentListener(this);
setSize(700,300);
setTitle("Set your own color..");
setVisible(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public void adjustmentValueChanged(AdjustmentEvent e)
{
int cr,cg,cb;
cr = r.getValue();
cg = g.getValue();
cb = b.getValue();
getContentPane().setBackground(new Color(cr,cg,cb));
}
public static void main(String [] args)
{
new AdjustmentEventDemo();
}
}
KeyEvent
| Event | Fired by | Listener interface | Handler method |
|---|---|---|---|
| java.awt.event.KeyEvent | All component | KeyListener | KeyTyped() KeyPressed() KeyReleased() |
Example of KeyEvent
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class KeyEventDemo extends JFrame
{
JLabel l1;
JTextField tf;
JTextArea ta;
JScrollPane sp;
KeyEventDemo()
{
setLayout(null);
l1 = new JLabel("Enter a text");
l1.setBounds(50,50,100,30);
tf = new JTextField(20);
tf.setBounds(170,50,150,30);
ta =new JTextArea (10,10);
sp = new JScrollPane(ta);
ta.setBounds(170,100,150,150);
add(l1);add(tf);add(ta);
setSize(300,300);
setVisible(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setTitle("KeyEvent Example");
tf.addKeyListener(new KeyAdapter()
{
public void keyTyped(KeyEvent e)
{
int x = e.getKeyChar();
ta.append(""+x);
}
});
}
public static void main(String[] args)
{
new KeyEventDemo();
}
}
Adapter class
In java.awt.event.* we have Listener interface called WindowListener which contains seven abstract methods. In the derived class implements WindowListener interface; it is mandatory for derived class to define all the methods even though the derived class is not required. If the derived class wants to define the required methods, it has to extend its corresponding adapter class called java.awt.event.WindowAdapter and this class contains null body deinition for WindowListener Therefore which Listener interface contains more than one undefined method for that Listener interfaces have the appropriate class whose general notation is XXXAdapter.
| Listener | Adapter |
|---|---|
| WindowListener | WindowAdapter |
| KeyListener | KeyAdapter |
| MouseListener | MouseAdapter |
| MouseMotionListener | MouseMotionAdapter |
MouseListener and MouseMotionListener
| Event | Fired by | Listener interface | Handler method |
|---|---|---|---|
| java.awt.event.MouseEvent | All component | MouseListener MouseMotionListener |
mouseClicked() mousePressed() mouseReleased() mouseEntered() mouseExisted() mouseDragged() mouseMoved() |
Example of MouseEvent
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class MouseEventDemo extends JFrame
{
int x1,y1,x2,y2;
MouseEventDemo()
{
setSize(300,300);
setVisible(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.addMouseListener(new MouseAdapter()
{
@Override
public void mouseClicked(MouseEvent e)
{
Graphics g = getGraphics();
x1 = e.getX();
y1 = e.getY();
g.drawString("The position you have clicked is : "+x1+" "+y1,100,100);
}
});
this.addMouseMotionListener(new MouseMotionAdapter()
{
@Override
public void mouseDragged(MouseEvent e)
{
Graphics g = getGraphics();
x1 = e.getX();
y1 = e.getY();
g.drawOval(x1,y1,10,10);
}
});
}
public static void main(String [] args)
{
new MouseEventDemo();
}
}
}
windowEvent
| Event | Fired by | Listener interface | Handler method |
|---|---|---|---|
| java.awt.event.WindowEvent | JDialog JFrame JWindow | WindowListener | windowOpened() windowClosing() windowClosed() windowIconified() windowDeiconified() windowActivated() windowDeactivated() |
Example of WindowEvent
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class WindowEventDemo extends JFrame
{
int x1,y1,x2,y2;
WindowEventDemo()
{
setSize(300,300);
setVisible(true);
setTitle("WindowEvent Example");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.addWindowListener(new WindowAdapter()
{
@Override
public void windowClosing(WindowEvent e)
{
System.exit(0);
}
});
}
public static void main(String[] args)
{
new WindowEventDemo();
}
}
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